The instant invention relates to pipe couplings; more specifically, the instant invention relates to pipe couplings which are especially useful for wells, such as water wells which may have for one reason or another corrosive substances therein.
Normally, when wells are drilled, wells require tubular casings in the form of lengths of pipe which are coupled to one another. For some applications, such as pumping out ground water which may include corrosive materials, it is necessary to use stainless steel pipes. Since stainless steel pipes are expensive, economic constraints dictate that the pipes should have walls as thin as is consistent with the task that the pipes are required to undertake. It is frequently difficult to join thin-walled pipes, simply because one cannot readily cut threads into the wall without reducing the thickness of the pipe below an acceptable dimension at its juncture with other pipes. Stainless steel pipes can be welded together. However, welding is frequently difficult in the field and results in a bead around the exterior of the joint which could interfere with a pipe sliding smoothly into a bore hole. This is especially the case where the pipe is hundreds of feet long and may tend to bow when inserted into a well hole. Accordingly, it is necessary to provide the pipe with some type of end coupling which is easy to join in the field and which will provide a smooth exterior surface to the entire casing.
The prior art does not adequately address this problem, as is evident from a number of United States patents directed to the art of joining pipes. For example, U.S. Pat. No. 3,493,854 to Fether discloses the concept of welding male and female members to pipes. However, the bead is applied externally which is not always suitable for well casing applications in which valuable pipe is used, such as stainless steel pipe which one may desire to remove from the well hole after pumping is complete. Moreover, this patent does not disclose structure for adequately sealing the coupling against leakage so that corrosive materials do not corrode the threads of the pipe and cause leakage, or cause the threads to bind so that the coupling cannot be easily disconnected. It is especially important to provide seals when the threads are square threads in that square threads have considerable play.
While the prior art discloses numerous structures used for purposes which supposedly solve various problems different from the problems solved by the instant invention, these structures are not combined with structure one would apply to address the problems of interest in the particular art under consideration. For example, U.S. Pat. No. 1,393,057 to Vollmer and numerous other patents disclose O-rings; however, the O-rings are not arranged in such a way as to facilitate adequate sealing of couplings for water well casings, wherein the couplings are very easy to join and separate yet are strong and leak-proof. Likewise, U.S. Pat. Nos. 4,192,533 to Blowse and 4,377,302 to Kohyama disclose O-ring seals, yet the particular thread arrangements are expensive to machine and do not result in a casing joint which is easy to couple by inexperienced workers.
Applicant's U.S. Pat. No. 4,735,444 discloses a solution to the problem. However, this patent relies on O-rings received in grooves to create seals. The grooves weaken portions of the couplings which already have relatively thin walls, increasing the likelihood of the coupling fracturing.